This invention relates to a secondary electron multiplication target for a secondary electron conduction type vidicon, and more particularly to an improved secondary electron emitting layer constituting this target.
The secondary electron multiplication target of the invention is effective particularly as a target for use in a secondary electron conduction type vidicon (hereinafter referred to as "SEC vidicon"). For a better understanding of the construction, function and effect of the invention, with reference to FIGS. 1 and 2 schematically showing a construction of the SEC vidicon to which the target of the invention is applied, the function or action of the SEC vidicon is hereinafter explained, and simultaneously with reference to FIG. 3 general physical phenomenon occuring during a process in which the particles forming a secondary electron emitting layer are deposited on a conductive support layer (a signal-drawing out support plate) is explained. In FIG. 1, a vacuum envelope 1 comprises a tubular glass portion 2, a face plate 3 for sealing the glass portion 2, and a stem portion 4. On the inner surface of the face plate 3 consisting of a transparent glass plate, a photocathode 5 is formed. At a substantially intermediate position between the face plate 3 and the stem portion 4 is disposed a secondary electron conduction target 6 (hereinafter referred to as "SEC target"), which comprises a metal ring 6a , a conductive thin film 6b (this film concurrently acting as an image signal-drawing out electrode) stretched over the metal ring 6a, and a secondary electron emitting porous layer 6c. Generally, the left side of the shown target 6 is termed an "image section" and the right side thereof a "scanning section". Within the scanning section are received an electron gun including a cathode electrode 7a, control electrode 7b and acceleration electrode 7c, a focusing electrode 8, a suppressing mesh electrode 9, and a field mesh electrode 10, while outside the scanning section is arranged a coil assembly 11 consisting of a focusing coil and a deflecting coil. Within the image section, electrodes 12a, 12b and 12c are received. The coupling relation between the target 6, the suppressing mesh electrode 9 and the electron gun is shown enlarged in FIG. 2. Examples of voltages to be applied to the photocathode 5, target 6, suppressing mesh electrode 9, and each of said electrodes are shown in FIGS. 1 and 2.
When, in the SEC vidicon having the foregoing construction the optical image of an object to be picked up is focussed on the photocathode 5, the photoelectrons emitted from the photocathode 5 are accelerated to 8 kv and transmitted through the conductive thin film 6b to enter the secondary electron emitting porous layer 6c. For this reason, a large number of secondary electrons are emitted from the porous layer 6c, and are moved, due to a voltage V.sub.T applied to the conductive thin film 6b, toward the conductive thin film 6b through the voids of the porous layer 6c. As a result, positive charge remains within the porous layer 6c. The distribution of the positive charge corresponds to the intensity of the optical image incident into the photocathode 5. At this time, when, in the scanning section, the secondary electron emitting layer or porous layer 6c of the target is scanned by scanning beam, said positive charge is neutralized to permit the secondary electrons to flow through a resistor 13 (see FIG. 1). Through drawing out the variation in voltage drop of the resistor 13 through a capacitor 14, an image signal I.sub.sig can be obtained. The suppressing mesh electrode 9 is given an appropriate potential necessary to prevent an excessive potential increase in the surface of the target on the scanning side, while the field mesh electrode 10 functions to land the scanning beam on the target surface at right angles thereto. Generally, the reason why the SEC vidicon is a highly sensitive image pickup tube is that the photoelectrons emitted from the photoelectric surface 5 are amplified within the target by said secondary electrons. When the signal current is represented by I.sub.sig and the photoelectron current by I.sub.pc, a gain G.sub.T within the target is expressed by the equation G.sub.T = I.sub.sig /I.sub.pc.
An attempt has been made to apply alkali halide such as KCl or MgO as material constituting the secondary electron emitting porous layer of the target of the SEC vidicon. In the case of using a porous layer consisting of KCl or MgO, or a target containing this porous layer, due to such layer having the below-mentioned drawbacks, a practical image pickup tube having satisfactory characteristics has not been obtained. The secondary electron emitting layer using KCl has the drawbacks that (1) the layer property is likely to be changed due to the layer having high moisture-absorption characteristics; (2) since the layer has a low melting point and low heat resistance, upon incidence of a strong light into the photoelectric surface heat is locally developed within the layer to change the property thereof, and further due to the temperature elevation during the baking step of the manufacturing process the layer property is changed; and (3) the KCl of the layer reacts with alkaline metal produced in the envelope upon forming the photocathode to change the porperty of the layer. The secondary electron emitting layer using MgO has the drawbacks that (1) the layer property is likely to be changed due to the action of alkaline metal produced in the envelope upon forming the photoelectric face; and (2) when, during operation of the image pickup tube, the target voltage V.sub.T is increased, due to an increase in electric field formed between the signal electrode 6b and the surface of the layer on the scanning side the secondary electron emitting layer thickness becomes small, or the particles constituting the layer are agglomerated to change the property of the layer, in which case, the change in property of the layer is prominent particularly when the layer is in a condition having moisture absorbed thereinto or having alkaline metal adhered thereto.
The structure change due to aggregation of particles constituting the secondary electron emitting layer leads to unevenness of secondary electron emission. As a result, a grainy picture appears on a picture screen. Therefore, the picture quality of the SEC vidicon whose secondary electron emitting layer is made of KCL or MgO is not good. Particularly when the gain G.sub.T is increased by raising the target voltage V.sub.T, such grainy picture prominently appears. And when the gain G.sub.T is increased up to 20 to 30, the SEC vidicon can no longer be used as a broadcasting image pickup tube. When, as above described, in addition to the fact that the picture quality of the SEC vidicon is originally not good, particles are excessively agglomerated during forming the secondary electron emitting layer, or this agglomerated condition is varied also after the formation of the emitting layer, the picture quality is further deteriorated. Furthermore, when the layer thickness becomes small with the variation of an agglomerated condition of particles, the duration of lag or after image becomes long. Furthermore, when agglomeration of the particles constituting the layer is promoted by incidence of photoelectrons or scanning beams during operation of the image pickup tube, the layer property is deteriorated permanently.
As a result of having experimentally studied the causes of production of said grainy picture in the image pickup tube using KCl or MgO as a target material, the present inventor has come to the following conclusions. That is to say, the secondary electron emitting layer is an aggregate of fine particles. Where the layer is made of KCl, these fine particles are obtained by evaporating the KCl in an atmosphere of inert gas (for example, N.sub.2, Ar, etc.). Where the layer is made of MgO, those fine particles are obtained by burning metallic magnesium in an atmosphere containing oxygen. In any of the above cases, said fine particles are grown in an atmosphere between an evaporation source (KCl) or a burning source (Mg) 16 received in a boat 15 shown in FIG. 3 and the support film 6b. The growth of said fine particles is effected in that region in the neighbourhood of the evaporation source 16 in which the temperature is higher than prescribed, and is stopped in that region apart from the evaporation source 16 in which the temperature is not more than prescribed. Where, after the growth of the fine particles is completed, some distance is left between the upper part of a growth region of the particles and the support film, the fine particles collide with each other in a space corresponding to said some distance, so that an aggregate of the fine particles is formed. The size of this aggregate depends upon, for example, the pressure of said atmosphere. Hereinafter, the fine particles obtained through the growth at said growth region of fine crystals formed from the evaporation source or burning source 16 at the initial stage are referred to as "primary particles", and the particles obtained through the agglomeration of the primary particles to reach the support film 6b are referred to as "secondary particles". The primary particle is generally monocrystalline (provided that in some cases it is polycrystalline), whereas the secondary particle is an aggregate of the monocrystalline particles and therefore is highly porous. The secondary particles are deposited on the support layer 6b. It should be noted, however, that the growth of fine crystals (primary particles) on the support film is not effected unlike the case with vacuum deposition, but that since, as above described, the secondary particle itself is porous and carried up to the support film 6b by a heated gas flow in the high temperature atmosphere, it fails to be uniformly deposited on the support film and this deposited layer contains tertiary particles formed by further agglomeration of the secondary particles. Due to containing these tertiary particles, the secondary electron emitting layer undergoes the formation therein of spatially rough and dense portions and the formation thereon of irregularities. The secondary and tertiary particles can be discriminated from each other by an electron microscope and optical microscope and their respective average particle size can be observed for measurement by the use of such microscopes. According to the present inventor's measurement, when burning metallic magnesium in the air, the average particle size of the primary particle was on the order of 1000 A to 1 .mu., the average particle size of the secondary particles was several microns, and the average particle size of the tertiary particles was scores of microns.
Through the above observation the following facts have been found. (1) The larger the average size of the primary particle, the larger the average size of the secondary and tertiary particles. (2) Even after the secondary electron emitting layer 6c has been deposited on the conductive thin film 6b, when the layer 6c absorbs moisture or alkaline metal is adhered to the layer 6c, said tertiary particles are formed. (3) Where the layer 6c is made of MgO, said tertiary particles are formed also in the case where the layer 6c is scanned with the target voltage V.sub.T increased up to a value greater than prescribed. (4) Where the average size of said tertiary particle exceeds 10 .mu.m, it becomes approximate to the diameter of a scanning beam, so that the unevenness with which the secondary electrons are emitted (when the particle size is different, the number of secondary electrons emitted is different based on the same amount of inciding photoelectrons) is read, or the unevenness with which the scanning beam is landed on the layer 6c occurs. These phenomena are causes of production of the above-mentioned grainy picture. (5) When the layer 6c having a thickness of, for example, microns, is scanned with a target voltage V.sub.T =30 volts, the electric field strength between the conductive layer 6b and the free surface of the layer 6c will reach about 3.times.10.sup.4 V/cm. This causes the porous layer 6c to contract in three dimensional directions. Therefore, the ternary particles are liable to be formed. The unevenness in the spacing of particles constituting the porous layer which emits secondary electrons leads to uneven emission of secondary electrons. The uneven emission results in a grainy picture. In the pickup tube having a target having a diameter of about 25 mm, the size of the visible image on the screen of a receiver is generally magnified 20 times as large as the target size. If the image on a receiver has an unevenness of 0.2 mm cycle, the unevenness becomes visible on the screen. If the unevenness exists all over the target, a grainy picture appears on the screen. The cycle of the unevenness on the target should therefore be limited to below 10 microns.
Accordingly, it is the object of the invention to provide a secondary electron multiplication target capable of eliminating the conventional drawbacks, through the above-mentioned experimental observation.